Apparatus for measuring the flow velocity of fluid within a conduit

ABSTRACT

Apparatus for measuring the flow velocity of fluid within a conduit. Sensing means external to the conduit and responsive to changes in the configuration of the conduit surface upon passage of a vortex and associated with electrical means measure the time interval during which the detectable property of the fluid passes between a plurality of preselected locations on the conduit. Fluid flow is not obstructed by installation of the sensing means. Thus, flow measurements, at a plurality of locations along the conduit can be economically obtained.

Gold

Feb. 5, 1974 APPARATUS FOR MEASURING THE FLOW VELOCITY OF FLUID WITHIN ACONDUIT Primary Examiner-Donald O. Woodie] Assistant Examiner-John P.Beauchamp 32-- MEANS INTEGRA- TlON' [76] Inventor: Stephen J. Gold, 148Fernhill Ave., Agent Buff; Jonathan Lafayette, La. 70501 Plaut [22]Filed: May 11, 1972 211 App]. No.: 252,440 [57] ABSTRACT Apparatus formeasuring the flow velocity of fluid within a conduit. Sensing meansexternal to the con- (g1 duit and responsive to changes in theconfiguration of the conduit surface upon passage of a vortex and asso[58] new of Search, 194 194 194 ciated with electrical means measure thetime interval during which the detectable property of the fluid passesbetween a plurality of preselected locations on 156] References and theconduit. Fluid flow is not obstructed by installa- UNITED STATES PATENTStion of the sensing means. Thus, flow measurements, 3,473,377 /1969Reinecke 73/194 C at a plurality oflocations along the conduit can beec- Beck et al. F onomically btained 3,688,106 8/1972 Brain 73/194 F X10 Claims, 8 Drawing Figures POWER |2 SOURCE 54 I8 3 j SENSINQ SENSINGMEANS MEANS 24 AMPLIFI- AMPLIFICA- 2 CATION \TION 6 MEANS MEANS I a n nu "I l t 1 r 1 CONTROL 28 MEANS I I4 I MULTlPLlf I CATION I E 4| MEANS40 I 1 b 44 46 E l 1 1 I E PAIENT FEB SISH SHEEI 2 0F 3 AMPLIFICATIONMEANS 22 --w- 0 MULTIPLICATION MEANS 28 FROM AMPLIFI- CATION MEANS TOMULTI- PLICATION MEANS PATENIEDFEB 5M4 SHEET 3 BF 3 FROM CONTROL MEANS3O /l06 /|OB PULSE DETECTING sTORING GENERATOR MEANS MEANS CLOCK RAMPPULSE GENERATOR TO INTEGRATION GENERATOR MEANS 32 FROM AMPLIFICATIONMEANs 2e AMPLIFICATION CONDITIONING COMPUTATION MEANS MEANs. MEANS T ICON19ROL INGREMENTING MEANs 3O MEANS /20 FROM FIRST INTEGRATINGDETECTING MEANs 32 MEANS sEcONO GATE DETECTING l MEANs T0 ANALOG CIRCUIT46 APPARATUS FOR MEASURING THE FLOW VELOCITY OF FLUID WITHIN A CONDUITBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to apparatus for measuring the velocity of fluid through aconduit and more particularly to sensing means external to the conduitand associated with electrical means for measuring the time intervalduring which a detectable property of the fluid passes between aplurality of preselected locationson the conduit.

2. Description of the Prior Art One of the most troublesome problems ofindustrial instrumentation is the difficulty of economically measuringthe velocity of fluid through a conduit. Apparatus which has beenproposed by prior art workers for such velocity measurements includes aplurality of sensors fixedly mounted in the conduit wall for contactwith the fluid, and electrical means associated with the sensors fortiming the passage of a detectable property of the fluid therebetween.Suchintemal sensing apparatus tends to obstruct the flow and is notespecially suitable for use in measuring the flow of fluids which arecorrosive, poisonous or radioactive. Installation of the sensors is noteasily effected without temporarily stopping flow within the conduit.Due to the fixed location of the sensors, flow measurements at aplurality of locations along the conduit cannot be economicallyobtained. For the above reasons,.apparatus of the type described hasgenerally resulted'in higher costs for measuring fluid flow than havebeen considered desirable for commercial applications.

SUMMARY OF THE INVENTION The present invention provides an economicalapparatus wherein the velocity of a fluid within a conduit and having aReynolds number sufficient to form vortices therein is measured byelectrical means associated with a plurality of sensing means on theexterior surface of the conduit. The apparatus has a power source forsupplying an electrical current. A first sensing means, adapted to bemounted on the exterior surface of the conduit and responsive to changesin the configuration of the exterior surface, transmits. an electricalsignal from the power source upon passage of a vortex thereby. A secondsensing means, adapted to be mounted on the exterior surface of theconduit downstream of the first sensing means'and responsive to changesin the configuration of the exterior surface, transmits an electricalsignal from the power. source upon passage of 'the vortex thereby. Thefirst and second sensing means are respectivelyconnected through aplurality of signal paths to electrical means for amplifying theelectrical signals and for computing and'indicating the flow velocity ofthe fluid.

The electricalmeans typically comprises amplification means for linearlyreproducing the wave forms of the signals from the respective sensingmeans, and computation means for solving the equation for a maximum off(u,v), where: K is a scaling constant so chosenas to make the outputsignal compatible with the indicating recording and controllingmechanisms of the computation means, T is the integrating time constantof the computation means, u(t) is the output signal of the first sensingmeans, v(t+ A t) is the output signal of the second sensing means, At isthe time interval for transit of the vortex between the first and secondsensing means, hereinafter called the vortex transit time, and At is afunction generated by the computer and is varied manually orautomatically to enable detection and recognition of the maximum off(u,v). Computation of the fluid velocity may be carried out digitallyby means of a digital computor or by analog computation means.

In operation of the apparatus, when fluid flowing within the conduit hasa Reynolds number sufficient to form a vortex therein, as in the orderof at least about 4,000, the vortex passes through the conduit at theaverage flow velocity of the fluid. The pressure exerted by the fluid ata given point on the interior surface of the conduit is momentarilydecreased during passage of the vortex thereby. Such pressure decreaseeffects a corresponding change in the configuration of the exteriorsurface of the conduit, which configurational change proceeds down theconduit at substantially the same velocity as the vortex. Each of thefirst and second sensing means detects the configurational change andtransmits an electrical signal from the power source to the electricalmeans upon passage of the vortex thereby. The electrical means amplifiesthe signals and computes the flow velocity of the fluid according to theequation described hereinabove.

In a specific embodiment, each of the sensing means is comprised of astrain gage of the metallic or semiconductor variety in series with anelectrical power source and coupled to an amplification means through acapacitor, so that electrical signals resulting from dynamic variationsof the strain gage are transmitted to the amplification means. The waveforms of the signals from the sensing means are linearly reproduced bythe amplification means and transmitted to a computation meanscomprising (1) multiplication means for multiplying together the outputsignal of the second sensing means v(t+At) with a timedelayed version ofthe output signal of the first sensing means u(t), (2) control means forvariably controlling the time interval At during which the signal fromthe first sensing means is delayed in transmission to the multiplicationmeans, and (3) integration means for integrating the output of themultiplication means over a period of time T to give its mean value,hereinafter called the cross-correlation function. When the transit timeinterval (Al) is equal to the time interval for transmission of theoutput signal from the sensor through the control means to themultiplication means, hereinafter called the controlled time intervalAt, the cross-correlation function reaches a maximum. The controlledtime interval At for the maximum value of the cross-correlation functionuniquely defines the vortex transit time. An electrical signal having amagnitude equal to the controlled time interval At is transmitted to asimple analog or digital circuit which computes and records or indicatesthe flow velocity of the fluid. The velocity of the fluid is obtained bydividing the distance between the first and second sensing means by thevortex transit time.

The apparatus of this invention has advantageous structural features.Since the sensing means and the electrical means are each external tothe conduit, fluid flow is not obstructed. Flow measurement ofcorrosive, poisonous or radioactive fluids is facilitated. The sensorscan be installed without stopping flow within the conduit, and flowmeasurements from a plurality of locations can be economically obtained.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fullyunderstood and further advantages will become apparent when referenceismade to the following detailed description of the preferred embodimentsof the invention and the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of apparatus for measuring theflow velocity of fluid in a conduit;

FIG. 2 is a schematic electrical diagram of the sensing means of FIG. 1showing its connection to a source of electrical power and toamplication means;

FIG. 3 is an isometric view of one form of a strain gage for use withthe sensing means of FIG. 2;

FIG. 4 is a perspective view of one embodiment of a control means;

FIG. 5 is a schematic electrical diagram of another embodiment of thecontrol means;

FIG. 6 is a diagrammatic representation of one form of amultiplification means;

FIG. 7 is a diagrammatic representation of one form of a signalconditioning means;

FIG. 8 is a diagrammatic representation of one form of a maximum seekingcircuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The fluid transferring conduitswith which the present invention can be used may be fabricated in anumber of diverse sizes and configurations. As a consequence, theinvention will be found to function with most varieties of suchconduits. For illustrative purposes, the invention is described inconnection with a substantially cylindrical conduit for transferring afluid therewithin. As used in this paragraph and elsewhere in thespecification and claims the term fluid includes compressible andnoncompressible substances tending to conform to the outline ofacontainer, such as homogeneous liquids, emulsions, slurries andsuspensions, gases and gas solid mixtures, and granular or pulverizedsolids and the like.

Referring to the drawings, the apparatus, shown generally at 10,includes a power source 12 connected to a computation means 14 throughtwo parallel signal paths, each path having a sensing means and incombination with the other path an electrical means, as described below.The first signal path includes a first sensing means 16 mounted on theexterior surface 18 of a cylindrical conduit 20 and connected throughline 34, amplification means 22 and line 36 to the computation means 14.The second signal path includes a second sensing means 24 mounted on theexterior surface 18 of the conduit 20 downstream of the first sensingmeans 16 and connected through line 38, amplification means 26 and line40 to the computation means 14. Each of the sensing means 16 and 24 isresponsive to changes in the configuration of the exterior surface 18 ofthe conduit 20. When fluid flowing within the conduit has a Reynoldsnumber sufficient to form vortices therein,

as in the order of at least about 4,000, preferably at least about10,000, the vortices pass through the conduit at the average flowvelocity of the fluid. The pressure exerted by the fluid at a givenpoint on the interior surface of the conduit is momentarily decreasedduring passage of a vortex thereby. Such pressure decrease effects acorresponding change in the configuration of the exterior surface of theconduit, which configurational change proceeds down the conduit atsubstantially the same velocity as the vortex. Each of the sensing means16 and 24 transmits an electrical signal to its respective amplificationmeans 22 and 26 upon passage of the vortex thereby. The wave forms ofthe signals from the sensing means 16 and 24 are amplified byamplification means 22 and 26, respectively. Upon exiting theamplification means 22 and 26 the signals from the sensing means 16 and24 are respectively transmitted to the computation means 14, which isprogrammed to solve the above-noted equation:

Computation of the fluid velocity may be carried out digitally by meansof a digital computer (not shown) or by analog computation means ashereinafter described.

FIG. 1 illustrates a computation means of the analog variety. Otherforms of the computation means can also be used. The computation meansshown in FIG. 1 should therefore be interpreted as illustrative and notin a limiting sense. Such means may comprise a multiplication means 28for multiplying together the output signal v(H-At) of the second sensingmeans 24 with a time delayed version of the output signal u(t) of thefirst sensing means 16, control means 30 for variably controlling thetime At by which the signal from the first sensing means 16 is delayedin transmission to the multiplication means 28, and integration means 32for integrating the output of the multiplication means 28 over a periodof time T to give the cross-correlation function.

Each of the amplification means 22 and 26 has a frequency responseranging from about 0.1 to 1,000 Hertz and a gain of sufficient magnitudeto elevate the signal voltages from the sensing means 16 and 24 to auseful level, as in the order of about I to 20 volts. Generallyspeaking, a gain of from about 10 to 10 is sufficient for this purpose.

The methods and means for generating the controlled time interval At arenumerous. For example, in one embodiment,'shown in FIG. 4, the controlmeans may be comprised of a substantially annular disc having an outerperipheral surface 82 of magnetic material and mounted on a support 84for rotation by motor 86 at a constant velocity. An amplified signalfrom amplification means 22 and line 36 is recorded on the disc 80 by arecording head 88 fixedly mounted on the support 84 adjacent theperipheral surface 82. The delayed signal is obtained from a pick-uphead 90 movably mounted on the support adjacent the peripheral surfaceof the disc. By mechanically varying the peripheral separation of therecording and pick-up heads 88 and 90, respectively, the time intervalduring which the amplified signal from the first sensing means isdelayed in transmission to the multiplication means can be variablycontrolled. In another embodiment, shown in FIG. 5, the control meansmay be comprised of a variably tapped delay line generally indicated at92 composed of inductors 94 and capacitors 96 so arranged and terminatedby resistor 98' as to constitute an artificial transmission line. Theseand other embodiments of the control means are intended to fall withinthe scope of the invention as defined by the subjected claims.

The amplified signal v(t+At) from the amplification means 26 and line 40and the delayed signal u(t+At') from the control means 30 and line 41are multiplied together by the multiplication means 28. Such means maybe of any suitable pattern such as, for example, a device making use ofthe Hall effect or a servo motor driven potentiometer. In still anotherembodiment, shown in FIG. 6, the multiplication means 28 can comprise apulse generator 100 for providing a regularly recurring series of pulseshaving a high state and a low state. The time duration of the high stateis varied in proportion to the magnitude of the signal u(z-l-At) fromthe control means 30. A ramp function generator 102 is provided forgenerating a regularly recurring signal in the form of a series of rampseach having a magnitude which increases linearly with time. Each of theramps is initiated concurrently with a pulse from pulse generator 100 bya clock pulse from clock pulse generator 104. The ramp terminates aftera preselected time interval of at least the same duration as the timeinterval during which the pulse has a high state. The maximum magnitudeof the ramp is equal to the magnitude of the signal v(t+At) from thesecond sensing means 24 and amplification means 26. Detecting means 106and storing means 108 are provided for detecting and storing themagnitude of the ramp signal at the instant at which the pulse returnsto a low state. The stored signal, which is the product of the outputsignal v(r+At) of the second sensing means 24 and the output signalu(t+At) from the first sensing means 16 is transmitted through line 42to the integration means 32.

The integration means 32 integrates the output of the multiplicationmeans over a period of time to give the cross-correlation function. Suchmeans may comprise a resistance/capacitance system such as an amplifierand capacitive feedback combination. The construction and operation ofthe integration means 32 is well understood by those skilled in the art.For this reason such means have been illustrated schematically.

When the transit time interval At is equal to the controlled timeinterval At, the cross-correlation function reaches a maximum. At thismaximum, the controlled time interval At uniquely defines the maximumvalue of the vortex transit time. An electrical signal having amagnitude equal to the controlled time interval At is transmittedthrough line 44 to an analog circuit 46 which computes and records orotherwise indicates the flow velocity of the fluid. Such velocity iscomputed in accordance with the equation: V= d/t where: V is the fluidvelocity, d is the distance between the first sensing means 16 and thesecond sensing means 24 and t is the vortex transit time.

In FIG. 2 there is shown schematically an electrical diagram of one formof the sensing means 16 and 24. Other forms of sensing means can also beused. Preferably, each of the sensing means is constructed in the samemanner. Thus the sensing means 24. can be constructed in the same manneras the sensing means 16,

which is described hereinafter in more detail. Such means may comprise astrain gage having a resistance element 48 of the metallic orsemiconductor variety in series with the power source 12 and coupled tothe amplification means 22 through a capacitor 50. The power source 12may be a primary or a storage battery having an electrical potential ofabout 6 volts. A resistor 52 having a resistance of approximately 10times the resistance element 48, as in the order of about 10,000 ohms isconnected in series between the battery and the resistance element 48.The resistance element 48 has a resistance ranging from about 50 to10,000 ohms, preferably from about to 1,000 ohms. A constant electricalcurrent from power source 12 and line 54 passes through the resistor 52,lines 56 and 58, the resistance element 48 and lines 60 and 62. Dynamicvariations in the configurations of the exterior surface 18 of theconduit 20 change the resistance value of the resistance element 48. Anelectrical signal from line 64 and capacitor 50 is transmitted throughline 34 to the amplification means 22. In this manner the dynamicvariations of the strain gage resistance are impressed upon the inputcircuitry of the amplification means 22. The capacitor 50 has acapacitance of sufficient magnitude that the frequencies of theelectrical signals within the range of interest of the amplificationmeans 22 are transmitted substantially without distortion. Such range ofinterest is predetermined and comprises the voltage alternationsexpected to result from passage of the vortex. Thus the capacitance ofthe capacitor 50 is generally in the order of about 0.01 to 100microfarads, preferably about 0.1 to 10 microfarads.

In FIG. 3 there is shown one form of the strain gage. Other forms ofsuch gage may also be used. The strain gage shown in FIG. 3 is intendedto be illustrative and should not be interpreted to limit the scope ofthe invention to the particular structure disclosed. Such strain gagecomprises at least one resistance element 48 of platinum, nickel,germanium, silicon or other suitable electrically resistive materialpermanently connected to a nonconductive support 68. The connection maybe obtained by encapsulating the resistance element 48 within a matrixof epoxy-impregnated glass fibers or by rigidly bonding the resistanceelement 48 to the upper surface 70 of the support 68. Lead wire 58 iselectrically connected to one end 72 of the element 48. The other end 74of the element 48 is electrically connected to lead wire 60. The lowersurface of the support is rigidly secured to the exterior surface 18 ofthe conduit 20 (shown in FIG. 1), by a suitable epoxy resin or the like.1

The apparatus 10 which has been disclosed herein can be modified innumerous ways without departing from the scope of the invention.

Interfering signals imposed upon the first and second signal paths canbe selectively filtered from the total signals of such paths by a signalconditioning means which reduces the magnitude of the interferingsignals. Such signal conditioning means can be connected in each of saidsignal paths (1) between the sensing means and the amplification means,(2) between the amplification means and the computation means, (3) ateach of locations l and (2), or (4) within each of the amplificationmeans. As shown in FIG. 7, the signal conditioning means 110 isconnected in each of the first and second signal paths between theamplification means 22, 26 and the computation means 14. The type ofsignal conditioning means selected and its particular location withinthe electrical means depends upon the amount and nature of theinterfereing signals to be filtered. For example, the magnitude ofinterfering signals impressed upon the output of the amplification meansby commercial power installations can be effectively reduced by aband-stop filter connected in each of said signal paths between theamplification means and the computation means and centered on thecommercial power frequency, typically 60 Hertz. Other types of signalconditioning means, such as a high-pass filter, a low-pass filter, aband-pass filter, or any combination thereof, can be used to reduce suchinterfering signals. The construction and operation of such signalconditioning means, as well as the selection of a particular typethereof, is well understood by those skilled in the art.

As mentioned hereinabove, computation of the fluid velocity may becarried out by a digital computer. Moreover, as shown in FIG. 8 the timecontrol means can include a maximum seeking circuit 120 forautomatically adjusting the controlled time interval At. The maximumseeking circuit is preferably in the form of a hill climbing controllerof the perturbation or other suitable variety having (I) incrementingmeans 122 adapted to alter the controlled time interval At by apredetermined increment of time, (2) first detecting means 124 fordetecting whether the output signal of the integration means 32 hasincreased or decreased and causing the incrementing means 122 to alterthe succeeding controlled time interval At by the predeterminedincrement of time so as to increase the integration output and (3)second detecting means 126 for causing gate 128 to transmit a signalfrom integrating means 32 to analogue circuit 46 when successivealterations made by the incrementing means 122 have alternatingdirections. These and other modifications are intended to fall withinthe scope of the invention as defined by the subjoined claims.

In operation, a change in the configuration of the exterior surface 18of the conduit 20 induced by a vortex of the fluid therein momentarilyalters the length and, hence, the resistance value ofthe resistanceelement 48 of the first sensing means 16. An electrical signal u(t) fromline 64 and capacitor 50 is transmitted through line 34 to theamplification means 22. The amplified wave form of the signal u(t) istransmitted through line 36 to the control means which variably controlsthe time At by which the signal u(t) is delayed in transmission to themultiplication means 28. When configurational changes induced by thevortex (not shown) are sensed by the second sensing means 24, anelectrical signal v(t) is transmitted through line 38 to theamplification means 26. The amplified wave form of the signal v(t+At) istransmitted through lines 40 and 38 to the multiplication means 28. Theoutput signal u(t+At) from the control means 30 and the output signalv(i t+At) from the amplification means are multiplied together by themultiplication means 28. The output signal from the multiplication means28 is transmitted through line 42 to the integration means 32 whichintegrates the signal over a period of time T to give thecross-correlation function. When the transit time interval is equal tothe controlled time interval At, the cross-correlation function reachesa maximum. The controlled time interval At uniquely defines the maximumvalue of the vortex transit time, and an electrical signal having amagnitude equal to the controlled time interval At is transmittedthrough line 44 to an analog circuit 46. The latter computes and recordsor otherwise indicates the flow velocity of the fluid according to theabove noted equation V d/t.

Having thus described the invention in rather full detail it will beunderstood that these details need not be strictly adhered to but thatvarious changes and modifications may suggest themselves to one skilledin the art. It is accordingly intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

I claim:

1. Apparatus for measuring the velocity of fluid within a conduit, saidfluid having a Reynolds number sufficient to form vortices therein,comprising:

a. a power source for supplying an electrical current;

b. first sensing means, adapted to be mounted on the exterior surface ofsaid conduit and responsive to changes in the configuration of saidexterior surface, for transmitting an electrical signal from the powersource upon passage of a vortex thereby;

0. second sensing means, adapted to be mounted on the exterior surfaceof said conduit downstream of said first sensing means and responsive tochanges in the configuration of said exterior surface, for transmittingan electrical signal from the power source upon passage of the vortexthereby;

d. said first and second sensing means respectively connected through aplurality of signal paths to electrical means for amplifying theelectrical signals and for computing and indicating the flow velocity ofthe fluid.

2. Apparatus as recited in claim 1, wherein each of the sensing meanscomprises a strain gage having a resistance element in series with saidpower source and electrically connected to said amplification meansthrough a capacitor.

3. Apparatus as recited in claim 2, wherein said electrical meansincludes amplification means for linear reproduction of said signals,multiplication means for multiplying together the signal from saidsecond sensing means and the signal from said first sensing means,control means for variably controlling the time interval during whichthe signal from said first sensing means is delayed in transmission tosaid multiplication means, and integration means for integrating theoutput of said multiplication means over a period of time to give thecross-correlation function of the equation where K is a scalingconstant, T is an integration time constant, u(t) is the output signalof said first sensing means, At is the time interval for transit of saidvortex between said first and second sensing means, and At is thecrossrcgrrelatiQnfunctign.

4. Apparatus as recited in claim 3 wherein said electrical meansadditionally includes analog circuit means coupled to the integrationmeans for computing and indicating the flow velocity of the fluidaccording to the formula V=d/t whereV is the fluid velocity, dis thedistance between said first and second sensing means and t is the timeinterval for transit ofthe vortex between said first and second sensingmeans.

5. Apparatus as recited in claim 4 wherein said electrical meansadditionally includes signal conditioning means connected in each of thesignal paths between each of said first and second sensing means andsaid computation means for reducing the magnitude of interfering signalsimposed upon such paths.

6. Apparatus as recited in claim 4, wherein said amplification means hasa frequency response ranging from about 0.1 to 1,000 Hertz and a gain inthe order of about 10 to 7. Apparatus as recited in claim 6 wherein saidcontrol means comprises a substantially annular disc having an outerperipheral surface of magnetic material and mounted on a support forrotation at a constant velocity, a recording head fixedly mounted onsaid support adjacent said peripheral surface and electrically connectedthrough amplification means to the output of said first sensing meansfor recording an amplified signal of said first sensing means on saiddisc, a pick-up head movably mounted on said support adjacent saidperipheral surface of said disc and electrically connected to saidmultiplication means, and means for varying the peripheral separation ofsaid recording and pick-up heads.

8. Apparatus as recited in claim 7, wherein said means for varying saidperipheral separation of said recording and pickup heads includes amaximum seeking circuit, whereby the time interval during which thesignal from said first sensing means is delayed in transmission to saidmultiplication means is automatically controlled.

9. Apparatus as recited in claim 6, wherein said control means includesa tapped delay line so arranged and terminated as to constitute anartificial transmission line.

10. Apparatus as recited in claim 8, wherein said multiplication meanscomprises a pulse generator for providing a regularly recurring seriesof pulses having a high state and a low state, means for varying thetime duration of said high state of each pulse in proportion to themagnitude of the signal from said control means, a ramp functiongenerator for generating a regularly recurring signal in the form of aseries of ramps, each of said ramps being initiated concurrently with apulse and terminating after a preselected time interval of at least thesame duration as the time interval during which said pulse has a highstate, the magnitude of each of said ramps increasing linearly with timeto a maximum equal to the magnitude of the signal from said secondsensing means.

1. Apparatus for measuring the velocity of fluid within a conduit, saidfluid having a Reynolds number sufficient to form vortices therein,comprising: a. a power source for supplying an electrical current; b.first sensing means, adapted to be mounted on the exterior surface ofsaid conduit and responsive to changes in the configuration of saidexterior surface, for transmitting an electrical signal from the powersource upon passage of a vortex thereby; c. second sensing means,adapted to be mounted on the exterior surface of said conduit downstreamof said first sensing means and responsive to changes in theconfiguration of said exterior surface, for transmitting an electricalsignal from the power source upon passage of the vortex thereby; d. saidfirst and second sensing means respectively connected through aplurality of signal paths to electrical means for amplifying theelectrical signals and for computing and indicating the flow velocity ofthe fluid.
 2. Apparatus as recited in claim 1, wherein each of thesensing means comprises a strain gage having a resistance element inseries with said power source and electrically connected to saidamplification means through a capacitor.
 3. Apparatus as recited inclaim 2, wherein said electrical means includes amplification means forlinear reproduction of said signals, multiplication means formultiplying together the signal from said second sensing means and thesignal from said first sensing means, control means for variablycontrolling the time interval during which the signal from said firstsensing means is delayed in transmission to said multiplication means,and integration means for integrating the output of said multiplicationmeans over a period of time to give the cross-correlation function ofthe equation
 4. Apparatus as recited in claim 3 wherein said electricalmeans additionally includes analog circuit means coupled to theintegration means for computing and indicating the flow velocity of thefluid according to the formula V d/t whereV is the fluid velocity, d isthe distance between said first and second sensing means and t is thetime interval for transit ofthe vortex between said first and secondsensing means.
 5. Apparatus as recited in claim 4 wherein saidelectrical means additionally includes signal conditioning meansconnected in each of the signal paths between each of said first andsecond sensing means and said computation means for reducing themagnitude of interfering signals imposed upon such paths.
 6. Apparatusas recited in claim 4, wherein said amplification means has a frequencyresponse ranging from about 0.1 to 1,000 Hertz and a gain in the orderof about 105 to
 107. 7. Apparatus as recited in claim 6 wherein saidcontrol means comprises a substantially annular disc having an outerperipheral surface of magnetic material and mounted on a support forrotation at a constant velocity, a recording head fixedly mounted onsaid support adjacent said peripheral surface and electrically connectedthrough amplification means to the output of said first sensing meansfor recording an amplified signal of said first sensing means on saiddisc, a pick-up head movably mounted on said support adjacent saidperipheral surface of said disc and electrically connected to saidmultiplication means, and means for varying the peripheral separation ofsaid recording and pick-up heads.
 8. Apparatus as recited in claim 7,wherein said means for varying said peripheral separation of saidrecording and pickup heads includes a maximum seeking circuit, wherebythe time interval during which the signal from said first sensing meansis delayed in transmission to said multiplication means is automaticallycontrolled.
 9. Apparatus as recited in claim 6, wherein said controlmeans includes a tapped delay line so arranged and terminated as toconstitute an artificial transmission line.
 10. Apparatus as recited inclaim 8, wherein said multiplication means comprises a pulse generatorfor providing a regularly recurring series of pulses having a high stateand a low state, means for varying the time duration of said high stateof each pulse in proportion to the magnitude of the signal from saidcontrol means, a ramp function generator for generating a regularlyrecurring signal in the form of a series of ramps, each of said rampsbeing initiated concurrently with a pulse and terminating after apreselected time interval of at least the same duration as the timeinterval during which said pulse has a high state, the magnitude of eachof said ramps increasing linearly with time to a maximum equal to themagnitude of the signal from said second sensing means.